Studies of coupled oscillators started in the 1600s, when the man who invented the pendulum clock set a pair of clocks side by side in a single case and noticed that they started ticking in unison. In mammalian cells, the machinery that controls the cell division cycle turns out to be similarly synchronized with the daily circadian clock, which allows cells to get on the same seasonal and day-night schedules. Feillet et al. imaged single live mammalian cells in culture and performed mathematical modeling. They showed that the daily circadian clock and the cell division cycle oscillate together at the same frequency. This may have clinical relevance: Circadian disruption is a risk factor for some cancers.